A Generalised Assessment of Working Fluids and Radial Turbines for Non-Recuperated Subcritical Organic Rankine Cycles

Abstract

The aim of this paper is to conduct a generalised assessment of both optimal working fluids and radial turbine designs for small-scale organic Rankine cycle (ORC) systems across a range of heat-source temperatures. The former has been achieved by coupling a thermodynamic model of subcritical, non-recperated cycles with the Peng-Robinson equation of state, and optimising the working-fluid and cycle parameters for heat-source temperatures ranging between 80 • C and 360 • C. The critical temperature of the working fluid is found to be an important parameter governing working-fluid selection. Moreover, a linear correlation between heat-source temperature and the optimal critical temperature that achieves maximum power output has been found for heat-source temperatures below 300 • C (T cr = 0.830T hi + 41.27). This correlation has been validated against cycle calculations completed for nine predefined working fluids using both the Peng-Robinson equation of state and using the REFPROP program. Ultimately, this simple correlation can be used to identify working-fluid candidates for a specific heat-source temperature. In the second half of this paper, the effect of the heat-source temperature on the optimal design of a radial-inflow turbine rotor for a 25 kW subcritical ORC system has been studied. As the heat-source temperature increases, the optimal blade-loading coefficient increases, whilst the optimal flow coefficient reduces. Furthermore, passage losses are dominant in turbines intended for low-temperature applications. However, at higher heat-source temperatures, clearance losses become more dominant owing to the reduced blade heights. This information can be used to identify the most direct route to efficiency improvements in these machines. Finally, it is observed that the transition from a conventional converging stator to a converging-diverging stator occurs at heat-source temperatures of approximately 165 • C, whilst radially-fibered turbines seem unsuitable as the heat-source temperature exceeds 250 • C; these conclusions can be used to inform expander design and selection at an early stage.

Simultaneous Cycle Optimization and Fluid Selection for ORC Systems Accounting for the Effect of the Operating Conditions on Turbine Efficiency

“…and by allowing p r to take values both above and below unity, and ε to vary between 0 and 1, it is possible to evaluate subcritical and transcritical cycles with or without recuperation. The validity of the ORC model for non-recuperated, subcritical cycles, based on the Peng-Robinson equation of state, has previously been confirmed by the authors (White and Sayma, 2018). It was observed that the optimal cycle parameters identified from the model were the same as those identified using an ORC model based on REFPROP, with the maximum deviation in the power output being <6%.…”

“…where the coefficients A, B, and C are constants that control the shape of the fluid's saturation dome, and thus whether the fluid has a saturated vapor line with a positive or negative gradient and is categorized as dry or wet, respectively. This has been demonstrated in our previous study (White and Sayma, 2018). Using Equations (1, 2) all of the required properties to analyse the thermodynamic cycle, namely the specific enthalpy h in J/mol, the specific entropy s in J/(mol K), and the specific volume v = 1/ρ in m 3 /mol, can be determined based on a vector of six fluid parameters:…”

“…Within this work the cubic Peng-Robinson equation of state is used to model the working fluid (Peng and Robinson, 1976), which allows a potential fluid to be defined in a more generalized way than using more sophisticated models, such as REFPROP (Lemmon et al, 2013), which, in turn, allows the working fluid to become a variable within the optimization process. The precise details on how the Peng-Robinson model is implemented within the ORC model are described in White and Sayma (2018), but in summary, the Peng-Robinson equation state is defined as:…”

“…The thermodynamic ORC model described in the same previous study (White and Sayma, 2018) is used here, although a number of modifications have been implemented. To summarize, the cycle is assumed to operate under a steady-state, and no heat losses or pressure drops are considered.…”

Experimental Study of an Organic Rankine Cycle Using n-Hexane as the Working Fluid and a Radial Turbine Expander

“…The efficiency was also increased from 10.5% for the single-stage turbine to 14.2% for the two-stage turbine. A generalized assessment of both optimal working fluids and radial turbine designs for a small-scale organic Rankine cycle was done by [25], using the Peng-Robinson equation of state to optimize the working fluid and cycle parameters in the temperature range of 80 to 360 • C. A correlation has been deduced which relates the heat source temperature to the critical temperature of the working fluid, which can be used to find the optimum working fluid for a given temperature. Using zeotropic mixtures of working fluids in an ORC will increase the performance of the ORC.…”